Not Applicable.
Not Applicable.
This invention relates to the use of Raman spectrometry in processes for polymerizing olefins and in methods of monitoring and controlling olefin polymerization processes, reactants and other components. More particularly, a Raman fiber optic probe may be placed in an olefin polymerization reactor, or before or after such a reactor, for Raman spectrometry analysis. The present processes and apparatus may employ low resolution Raman spectrometry and measurement of liquid phase and/or gas phase components of an olefin polymerization process. The present processes and apparatus allow for quantitatively monitoring the olefin polymerization process in situ and constitute an improvement over gas chromatographic analysis conventionally employed in monitoring olefin polymerization reactions.
Olefin monomers, such as ethylene and propylene, can be polymerized to form polyolefins. For example, ethylene or propylene may be homopolymerized to form polyethylene and polypropylene, respectively, or they may be copolymerized together or with higher 1-olefins such as 1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene and others. Processes for manufacturing polyolefins typically employ catalyst systems comprising one or more catalytic metal compounds, typically transition metals, perhaps together with a co-catalyst and/or a support such as alumina or silica. Different types of olefin polymerization processes are available. For example, olefins may be polymerized in homogeneous processes or in heterogeneous processes.
One type of polymerization process employs a slurry as the reaction mixture. In slurry polymerization processes, solid olefin polymers such as polypropylene, polyethylene and copolymers, are formed under polymerization conditions that include a slurry as the reaction mixture. The slurry comprises the solid olefin polymer particles suspended in a liquid diluent that is inert in the polymerization reaction and in which the polymer is insoluble under polymerization conditions. Typically, slurry polymerization processes are conducted in a relatively high-pressure continuous reactor, such as a loop reactor. Such reactors may be operated at pressures of about 600 psi, for example, and at temperatures of about 60 degrees C. to about 100 degrees C. In many situations, slurry polymerization processes are relatively more commercially desirable than other polymerization processes.
In the slurry polymerization process, components such as one or more monomers, a diluent, and a catalyst system and possibly other reactants (such as, for example, comonomers or hydrogen) are introduced to the polymerization reactor to form a reaction mixture. The reaction mixture is maintained under polymerization conditions for formation of polyolefin. After a suitable period, the slurry or a portion thereof is discharged from the reactor through a product take-off line into a settling leg. The solid polyolefin settles out from the slurry, leaving a clear liquid comprising diluent and reactants such as the monomer. The clear liquid and solid polyolefin then may then re-mix as they are transferred to one or more separation chambers or flash tanks where, for example, they are flashed to a low pressure such as about 15 or 20 psi. Some slurry loop polymerization equipment includes both a high pressure flash tank and a low pressure flash tank. Further information and details of slurry polymerization processes and loop reactors, including examples of suitable reaction conditions as well as control schemes for other important variables, such as solids concentration and production rate, can be found in U.S. Pat. No. 3,998,995 and U.S. Pat. No. 3,257,363, which are incorporated herein by reference.
It is desired to monitor and control the polymerization reaction so that one may obtain polyolefins having particular properties. Obtaining particular properties in a polyolefin may be done by control of the component concentrations or ratios during the polymerization process. Small changes in components can affect the properties of the final polyolefin product. Control of the concentration of olefin monomer, and if present, comonomer and hydrogen, is required to ensure reliable finished polyolefin product properties. Other important control parameters may include the degree of polymerization, molecular weight, or size of the polymer chain. Therefore, it is desirable to monitor the olefin polymerization process by determining monomer content and, when one or more co-monomers are present, by determining co-monomer content(s). It may also be desirable to determine diluent content and product content.
Current methods of monitoring slurry polymerization processes and the components (reactants, products, and diluent) in such processes are less than optimal for several reasons. In loop polymerization processes, monomer and co-monomer content have typically been determined by gas chromatography (xe2x80x9cGCxe2x80x9d) analysis of the flash gas, that is, the gas released at one of the flash tanks where pressure is released. For example, U.S. Pat. No. 3,257,363 discloses methods of controlling the composition of the reaction mixture in a loop polymerization reactor wherein a gas chromatographic analyzer may be used to determine the amounts of ethylene and 1-butene reactants from a polymer-free off-gas line or with a sample stream from anywhere in the reaction system.
However, monitoring of the olefin polymerization process by analysis of the flash gas is less than optimal for several reasons. One reason is the amount of time for such analysis. If an analysis takes too much time, it generally has less value for monitoring, controlling or adjusting the olefin polymerization process. Also, another concern arises when the polymerization equipment includes more than one flash chamber, such as a high pressure flash chamber and a low pressure flash chamber. In such instances, gas chromatographic analysis of flash gas takes more time and is potentially less accurate when both high pressure and low pressure flash tanks are in operation.
While the contents of the olefin polymerization reactor may be determined by removing a small sample for analytical testing in a remote laboratory, this is less favorable than monitoring in situ. It may be dangerous and difficult to remove a sample from a hot process stream, and there are risks that the sample may not be representative of the overall reactor contents or that removing the sample may alter the sample. Sampling is time-consuming, and delay may cause the sample not to be representative of the reactor contents. A significant amount of material may be produced in the time required to remove, prepare, and analyze a sample. The analytical data obtained from the delayed sample is therefore of limited value for proactive process control. Furthermore, additional processing of the extracted sample may be required yet is undesirable.
A preferred method of monitoring the polymerization process would monitor the process as it happens, or as soon thereafter as practical. It is also preferable that an analysis method be performed in situ, as opposed to being performed on samples removed from the polymerization equipment. An in situ method would reduce the need to remove samples from the production environment, improve safety, and yield faster measurements. However, there are obstacles to providing in situ on-line chemical information in a process environment. The analytical method must be sufficiently accurate and precise under hostile physical and chemical conditions. The analytical method must be capable of remote detection and analysis.
Analyses of slurry olefin polymerization processes in situ, that is, within a slurry loop polymerization reactor or associated equipment, have been difficult if not impossible to do, since such olefin polymerization reactions are carried out at high pressures. However, spectrophotometric apparatus such as a spectrograph and a radiation source can be situated in a location remote from the polymerization reactor that is to be analyzed in situ, the sampling site being connected to the apparatus by radiation conduits comprising fiber optic cables.
Raman spectrometry can provide qualitative and quantitative information about the composition and/or molecular structure of chemical components. Raman spectrometry is based upon the vibrational energy of a compound. A sample is irradiated, preferably by a monochromatic light source, and the scattered light is examined through a spectrometer using photoelectric detection. Most of the scattered radiation has the wavelength of the source radiation, which is referred to as Rayleigh scattering. However, the scattered radiation also comprises radiation at shifted wavelengths, which is referred to as Raman scattering, which occur at different wavelengths due to molecular vibrations. The difference in wavelengths between the source radiation and radiation affected by molecular vibrations is commonly referred to as the Raman shift. Even if monochromatic light is used as the source radiation, the Raman spectrum will comprise scattered light spread across a wavelength band. The Raman shift or Raman spectrum conveys compositional and molecular information regarding the component in the sample. The Raman spectrum is extremely weak compared to the Rayleigh spectrum.
Not all substances are measurable by Raman spectrometry. There must be a change in polarizability during molecular vibration of a substance in order for that substance to be Raman active.
There are several factors that have favored the use of gas chromatography as an analytical method over Raman spectrometry with olefin polymerization processes. In general, it is recognized by those familiar with Raman spectrometry that the presence of solid particles in a solution to be analyzed will significantly reduce the Raman shift observed. In particular, slurry olefin polymerization processes include solid polyolefin particles in the slurry. Also, the reactants and products in olefin polymerization processes may have peaks in their respective Raman spectra that are relatively close together, such as when ethylene is employed as a monomer and hexene is employed as a co-monomer. For example, ethylene and hexene produce similar peaks in their Raman spectra. Ethylene exhibits a peak at 1620 cmxe2x88x921 while hexene exhibits a peak at 1640 cmxe2x88x921. As a result, it may be difficult to distinguish between ethylene and hexene, and there is likely to be some overlap in certain peaks. Thus, it would appear necessary to employ high resolution Raman spectrometry equipment to analyze the components of certain olefin polymerization processes. Furthermore, Raman spectrometry equipment, particularly high resolution Raman spectrometry equipment, is relatively expensive, which would generally discourage its use with industrial processes. Gas chromatography equipment has historically been less costly than high resolution Raman spectrometry equipment. Furthermore, gas chromatography sampling systems are well established. Also, gas chromatography equipment tends to provide information that is more readily usable, whereas Raman spectrometry equipment tends to produce information that requires additional analysis. Engineers and operators tend to prefer equipment that provides a relatively simple reading rather than a spectrum.
International Application No. PCT/AU86/00076, which is incorporated herein by reference, discloses monitoring the presence or concentration of one or more chemical components by using Raman scattering. Optical fibers are used to direct electromagnetic radiation to and from the monitored environment, so that the Raman detector may be remote from the monitored environment. It is stated that the Raman monitoring method is applicable to gases, liquids and solids, though no particular chemical components are disclosed as being monitored. It is also stated that it is necessary to examine the intensity of the scattered light at selected characteristic wavelengths. A band pass filter system is used, which has a series of narrow band pass interference filters each having a band pass between 100 cmxe2x88x921 and 400 cmxe2x88x921. Each filter is chosen to give maximum transmission of the Raman scattering of a particular component to be analyzed. This international application does not disclose the use of Raman spectrometry to monitor an olefin polymerization process, or to measure olefin monomers. The international application does not disclose a method of monitoring the presence or concentration of more than one chemical component when those components have overlapping Raman spectra or event that such monitoring is possible.
U.S. Pat. No. 5,652,653, which is incorporated herein by reference, discloses a method of on-line quantitative analysis of chemical compositions by Raman spectrometry. The method comprises simultaneously irradiating the monitored chemical composition and a reference material. The method applies predetermined calibration means to the standard Raman spectrum of the analyzed chemical composition to ascertain the chemical composition. The method is used for analyzing a polyester manufacturing process. A polyester manufacturing process generally has a liquid reaction mixture that does not include solids or a slurry. The patent discloses the construction of constitution-intensity correlation (CIC) multivariate calibration means. This is done by comparing a plurality of peaks at different wavelengths in the Raman spectra, which are preferably standard spectra, with a plurality of chemical compositions of known concentrations. The wavelengths selected for construction of a CIC depend on the spectral characteristics of the particular component whose concentration in a chemical composition is to be determined. For each component whose in situ concentration in the composition is desired to be monitored at any given time, a separate CIC calibration is prepared.
U.S. Pat. No. 4,620,284, which is incorporated herein by reference, relates to qualitative and quantitative analysis using Raman scattering for substances in gaseous, liquid and solid form to provide numbers, rather than spectra, denoting the amounts of the substances present. It is disclosed that reference spectra are used to establish a relationship between spectra region areas and concentrations of substances, and that composite reference spectra may be prepared. The patent discloses a hydrocarbon analyzer dedicated to xe2x80x9cPNAxe2x80x9d analysis as a particular embodiment, which determines the composition of a hydrocarbon in terms of three groups: paraffins, napthlanes, and aromatics. Among the prior art disclosed in that patent is work accomplished using the Raman effect to analyze hydrocarbons, including an article entitled xe2x80x9cDetermination of Total Olefins and Total Aromatics.xe2x80x9d Similarly, an article entitled xe2x80x9cLow-Resolution Raman Spectroscopy,xe2x80x9d Spectroscopy 13(10) October 1998, discuss Raman spectrometric analysis of mixtures of organic liquids as well as petroleum products.
However, it is believed that Raman spectrometry has not been previously employed to monitor an olefin polymerization process.
The present processes, methods, and apparatus differ from prior processes and apparatus for monitoring chemical components with Raman spectrometry in that Raman spectrometry is applied to olefin polymerization processes. Monitoring of olefin polymerization processes by Raman spectrometry is distinguishable at least because Raman spectra of the various components may overlap, and other factors have made other analytical methods appear to be superior. For example, the presence of solid polyolefin particles in the reaction mixture of some polymerization processes would discourage the use of Raman spectrometry for analyzing such reaction mixtures.
A process for olefin polymerization in a slurry comprising solid polyolefin and a diluent is provided. The process comprises the steps of contacting in a reaction mixture under slurry polymerization conditions: (i) at least one reactant including at least one olefin monomer and optionally at least one comonomer and optionally hydrogen and (ii) a heterogeneous catalyst system comprising one or more catalytic metal compounds and one or more co-catalysts; (b) making a polyolefin; and (c) monitoring the process by using Raman spectrometry equipment to provide an output signal representative of one or more of reactants or the polyolefin; and (c). The output signal is generally representative of a concentration of either one of the reactants or the products, though the signal that comes directly from the Raman probe will be converted to a concentration value as described herein to provide the output signal.
The process may further comprise the step of adjusting the olefin polymerization process in response to the output signal provided by the Raman spectrometry equipment. The olefin polymerization process can be adjusted by adjusting the concentration within the reaction mixture of at least one chemical component with the reaction mixture. Alternatively, the olefin polymerization process can be adjusted by adjusting one or more polymerization conditions selected from the group consisting of polymerization temperature, polymerization pressure, withdrawal of the reaction mixture from the reactor, and circulation rate of the reaction mixture within the reactor. The Raman spectrometry equipment is operatively connected to a Raman fiber optic probe that is in contact with the reaction mixture or the polyolefin.
In some preferred embodiments, the monomer consists of ethylene. In other preferred embodiments, a comonomer is present in the reaction mixture so that it is contacted with the ethylene, and the comonomer is selected from the group consisting of 1-butene, 1-pentene, 4-methyl-1-pentene, and 1-hexene.
The monitoring may be done using Raman spectrometry equipment to analyze effluent, such as the effluent from a loop polymerization reactor.
As another aspect, a method for monitoring and controlling an olefin polymerization reaction is provided. The method comprises (a) contacting components of a reaction mixture in a polymerization reactor under polymerization conditions, where the components comprise a monomer, a diluent, and a catalyst system; (b) using Raman spectrometry equipment to obtain a Raman spectrum; (c) obtaining a concentration of at least one component based upon the Raman spectrum; (d) adjusting at least one polymerization condition in response to the concentration of the component. The method may also comprise obtaining a Raman spectrum of the reaction mixture, and determining the concentration of at least one component through the use of a calibration model. In preferred embodiments, the method further comprises, prior to step (a), the step of developing the calibration model using partial least squares analysis.
As yet another aspect, an apparatus for olefin polymerization is provided. The apparatus comprises polymerization equipment comprising a polymerization reactor for slurry polymerization of one or more olefins, wherein the slurry comprises solid polymer particles and a diluent; at least one inlet to the reactor for providing chemical components of the polymerization; at least one outlet from the reactor for removing product from the polymerization reactor; at least one Raman probe located in the polymerization equipment, where the Raman probe provides an output signal; Raman spectrometry equipment located outside the polymerization equipment and operatively connected to at least one Raman probe.
The olefin polymerization apparatus may further comprises a computer that receives a signal from Raman spectrometry equipment. The computer can be operatively connected to flow control equipment for adjusting a concentration of at least one of the chemical components or the product. Alternatively, the computer can be operatively connected to equipment for adjusting one or more of polymerization conditions selected from the group consisting of polymerization temperature, polymerization pressure, withdrawal of the reaction mixture from the reactor, and circulation rate of the reaction mixture within the reactor. The computer may comprise a calibration model for converting Raman spectra to at least one concentration of one or more of chemical components or of product.
The Raman probe may be a Raman fiber optic probe disposed in the outlet or in polymerization reactor. The Raman probe can be operatively connected to the Raman spectrometry equipment by fiber optic cable.
In the present processes, methods and apparatus, low resolution Raman spectrometry equipment may be used, such as, for example, Raman spectrometry equipment having a resolution of about 15 wavenumbers to about 30 wavenumbers.